Cell survival is regulated by various extracellular signals such as growth factors, cytokines, and extracellular matrices (ECMs) through cell surface receptors.
Two major signal transduction pathways regulate this signal transduction process from the cell surface in to the cell nucleus. One of these is the Ras signal transduction pathway, and the other is the Phosphatidylinositol 3 kinase (PI3K) pathway. The PI3K pathway is activated by cell surface receptors, or indirectly activated by Ras. The present invention focuses on the Ras signal transduction pathway.
The mitogen-activated protein kinase (MAPK) cascade composed by three kinases, specifically Raf, MEK (MAPK kinase or extracellular stimulus regulated kinase (ERK) kinase), and ERK, is a key module for the Ras signal transduction pathway. This cascade is initiated by the activation of Ras, and plays an important role in adjusting cell proliferation, differentiation, and transformation in response to extracellular signals (Non-Patent Documents 1 to 7).
Ras activation is regulated by the interplay between GTP exchange factors (GEFs) and GTPase activating proteins (GAPs) (Non-Patent Document 8). GEFs activate Ras through Ras-GTP complex formation, and GAPs inactivate Ras through Ras-GDP complex formation. Ras is activated by the action of extracellular signals, such as growth factors, on cell surface receptors, or by mutation of Ras itself. Mutations of Ras have been observed in many human cancer cells. Ras is known to be constitutively activated (GTP complex) by mutation to play an important role in human cancer cell proliferation.
Activated Ras interacts with Raf-1, which is a serine-threonine protein kinase, and activates Raf-1 (Non-Patent Documents 9 and 10).
Activated Raf-1 then phosphorylates and activates MEK1 and MEK2. This phosphorylation takes place at two MEK serine residues (Ser218 and Ser222) (Non-Patent Documents 11 to 15).
MEK is a dual specificity kinase, and the activated MEK phosphorylates ERK1 and ERK2 at the tyrosine (185) and threonine (183) residues (Non-Patent Documents 16 and 17).
Phosphorylation of ERKs by MEK not only activates ERKs but also translocates them to the nucleus.
Consequently, the activated ERKs (MAPK) activate various substrates in the cytoplasm and nucleus, such as transcription factors, and cause cellular changes (proliferation, differentiation, and transformation) in response to extracellular signals.
MEK has high substrate specificity, and no substrates other than ERK1 and ERK2 have been found to be phosphorylated by MEK (Non-Patent Document 18).
These unique MEK characteristics, such as high substrate specificity (ERK1 and 2 are the only substrates) and dual specificity (phosphorylates tyrosine and threonine), are not so commonly observed in other kinases, indicating that MEK has a central role in integrating signals into the MAPK pathway.
Constitutive activation of the MEK/MAPK pathway has been shown to be related to a relatively wide variety of neoplastic phenotypes of cancer cells (Non-Patent Documents 19 to 21).
Furthermore, constitutive activation of MEK is known to result in cell alteration (transformation) (canceration) (Non-Patent Documents 22 and 23).
In addition, studies using MEK inhibitors (such as PD98059) have shown that inhibition of MEK not only impairs proliferation of cells, but also has an impact on (interferes with) various cellular phenomena comprising differentiation of cells, apoptosis, and angiogenesis (Non-Patent Documents 24 to 31).
Thus MEK, which is one of the major mediators of the MAPK cascade, may be a potential target for therapeutic agents for treating diseases caused by abnormal cell proliferation.
To date, many compounds for MEK inhibition have been reported (Patent Documents 1 to 31). However, some of these compounds have problems with solubility or metabolic stability. Other compounds have problems with PK difference. For example, compound CI-1040 (Patent Document 6, Example 95), shown below, has been reported to be a MEK inhibitor. However, the results of its phase I clinical trials, which were reported at the 2002 American Society of Clinical Oncology Annual Meeting (Non-Patent Document 32), indicate problems of quick in vivo inactivation by hydrolysis, high lipid solubility and low water solubility, and large differences in pharmacokinetic parameters between patients.
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